1. Field of the Invention (Technical Field)
A new low-flow, low-power torch has been developed which utilizes laminar coolant gas flows. The laminar flow torch (LFT) is constructed by the addition of a machined insert between the outer and intermediate tubing of a conventional turbulent flow torch (TFT). This configuration has been demonstrated to provide both greater intensity signal and improved signal to noise ratio in comparison to a TFT at relatively low-power and low-flow operation conditions. The LFT demonstrated an increase in the intensity of the detected response of as much as a factor of ten for calcium ion. This LFT design has displayed excellent potential for use as a low-power, low-flow inductively coupled plasma torch for atomic emission spectroscopy.
2. Description of the Related Art Including Information Disclosed under 37 C.F.R. .sctn..sctn.1.97-1.99 (Background Art)
Many efforts have been made to improve the analytical performance of inductively coupled plasma (ICP) torches with lower argon gas consumption rates and lower applied radio frequency (rf) power requirements. These efforts have included reducing the overall dimensions of the torches (See "Design and Construction of a Low-Flow, Low-Power Torch for Inductively Coupled Plasma Spectrometry", R. Rezaaiyaan, et al., Allied Spectroscopy, Vol. 36, p. 627 (1982)), modification of current torch dimensions (see Rezaaiyaan, et al., ibid.; "Development and Characterization of a Miniature Inductively Coupled Plasma Source for Atomic Emission Spectrometry", R. N. Savage, et al., Anal. Chem., Vol. 51, p. 408 (1979); and "Development and Characterization of a 9-mm Inductively-Coupled Argon Plasma Source For Atomic Emission Spectrometry", A. D. Weiss, et al., Anal. Chem., Vol. 124, p. 245 (1981)), enhanced cooling efficiency of the torch (see "Reduction of Argon Consumption by a Water Cooled Torch in Inductively Coupled Plasma Emission Spectrometry", G. R. Kornblum, et al., Anal. Chem., Vol. 51, p. 2378, (1979); and "Water-Cooled Torch for Inductively Coupled Plasma Emission Spectrometry", H. Kawaguchi, et al., Anal. Chem., Vol. 52, p. 2440 (1980)), and the use alternate coolant media (e.g., air, water, or radiative cooling). (See "A New Reduced-Pressure "ICP Torch", C. J. Seliskar, et al., Applied Spectroscopy, Vol. 39, p. 181 (1985); "Determination of Metals in Xylene by Inductively Coupled Air Plasma Emission Spectrometry", G. A. Meyer, Spectrochim. Acta, Part B, Vol. 42B, p. 201 (1987); and "A Radiatively Cooled Torch for ICP-AES Using 1 1 min.sup.-1 of Argon", P. S. C. van der Plas, et al., Spectrochim. Acta, Vol. 39B, p. 1161 (1984)).
Typically, ICP torches have incorporated tangential flows for stabilization of the discharge. Optimization studies have indicated the constriction of the inner diameter of the gas inlet tubes of the torch to be a desirable feature in the construction of a low-power, low-flow torch (see Rezaaiyaan, loc. cit.; "Analytical Characteristics of a Low-Flow, Low-Power Inductively Coupled Plasma", R. Rezaaiyaan, et al., Anal. Chem. Vol. 57, p. 412 (1985)); and "Interferences in a Low-Flow, Low-Power Inductively Coupled Plasma", R. Rezaaiyaan, et al., Spectrochim. Acta, Part B, Vol. 40B, p. 73, (1985)). Constriction of the inlet tubing was proposed to result in a higher gas velocity of the gas vortex used to stabilize the plasma within the torch. However, this vortex gas flow pattern has been proposed to be the source of a 200-300 Hz component of the noise-power spectrum of the emission from an ICP (see R. M. Belchamber, et al., Spectrochim. Acta, Part B, Vol. 37B, p. 17, (1982)). Davies and Snook (J. Anal. Atom. Spectrosc., Vol. 1, p 195 (1986). and Analyst, Vol. 110, p. 887 (1985)) have recently described a torch design which demonstrated increased linear dynamic range and a reduction in the measured noise by incorporating laminar flow introduction of the coolant gas at the base of the torch.
An alternative design of a laminar flow torch (LFT) has been developed in accordance with the invention. The design incorporates several features which have been determined to improve the analytical performance of the torch with reductions in the rate of argon gas consumption and the required rf power level. It is based on the principle that a thin, ordered coolant gas flow along the inner wall of the plasma torch will be sufficient to keep the plasma fire ball away from the torch wall and to remove the heat generated in the plasma discharge. By arranging the coolant in a highly ordered thin layer, the operation of a low-power, low-flow, low-noise plasma torch has been achieved.